1. Technical Field
The present invention relates to an oscillator that uses a piezoelectric element such as crystal, particularly to a temperature-compensated piezoelectric oscillator that allows temperature compensation of frequencies with a simple circuit configuration, and that is suitable for integrated circuits.
2. Related Art
In recent years, the requirements for piezoelectric elements, for instance oscillators that use crystal resonators, have been increasingly demanded not only for frequency stability, but also for a lower price and smaller oscillators. Further, as the digitalization of communication system progresses, the improvement of carrier-to-noise ratio (C/N property) characteristics, which has not posed a problem in the past, is now sought after. Output frequencies of an oscillator changes with various causes. Even in crystal oscillators that have relatively higher stability in frequencies, there is a frequency deviation caused by a change in the surrounding temperature, the power source voltage, or the output load, and the like. There are various ways to cope with it. For example, there is a temperature-compensated crystal oscillator (hereafter referred to as “TCXO”) that copes with temperature change, in which: a temperature-compensated circuit is added to the crystal oscillator; the load capacity during oscillation loops is changed; and the load capacity is controlled in accordance with the temperature change so as to balance out the temperature-frequency characteristic unique to the crystal resonator.
FIG. 15A is a circuit diagram of a TCXO, conceivably provided by the same inventor. The TCXO shown in this example has a direct-current-stopping fixed capacitor C3, a temperature-compensated circuit 61, and a crystal resonator X connected serially to a Colpitts oscillation circuit 60. This temperature-compensated circuit 61 is organized including: a series circuit where a low-temperature compensation MOS capacitor ML and a sensitivity-adjusting fixed capacitor C4 are connected serially; and a parallel connection between a high-temperature compensation MOS capacitor MH and the series circuit. The polarities of the low-temperature compensation MOS capacitor ML and the high-temperature compensation MOS capacitor MH are opposite to each other. Moreover, a low-temperature section control voltage signal VL is supplied via an input resistor R4 to the mid-connection point between the anode terminal side of the low-temperature compensation MOS capacitor ML and the sensitivity-adjusting fixed capacitor C4, and a high-temperature section control voltage signal VH is supplied via an input resistor R5 to a gate terminal side of the high-temperature compensation MOS capacitor MH. Further, a reference voltage signal VREF is supplied via an input resister R6 to the gate terminal side of the low-temperature compensation MOS capacitor ML and to the anode terminal side of the high-temperature compensation MOS capacitor MH.
FIG. 15B includes graphs of the temperature-compensated voltages of the TCXO. The temperature compensation of the TCXO, according to aspects of the invention, utilizes a MOS varactor in order to perform a frequency-temperature compensation of the crystal resonator X. Since the capacitance deviation of the MOS varactor over temperature is similar to a behavior of a cubic function, the temperature compensation may be performed in a manner where the voltage applied to the MOS varactor behaves like a linear function over temperature. However, since the frequency-temperature characteristic of crystal resonators are subject to fluctuation, the temperature compensation with the MOS capacitance deviation does not always result in an ideal compensation curve, and the precision of the temperature compensation is not high. Therefore, the crystal resonators do not provide enough performance for the use of reference oscillators that require high stability in frequency, for instance, a GPS receiver.
FIG. 16B includes graphs expressing the temperature characteristic of the TCXO in FIG. 15. A solid line 62 (FIG. 16A) indicates the temperature characteristic of the crystal resonator X, and a dotted line 63 indicates the frequency variable characteristic over temperature, in the case where the temperature-compensated circuit 61 is controlled in the control voltage shown in FIG. 15B. As observed in the graphs, if the temperature-compensated circuit 61 is controlled with the control voltage shown in FIG. 15B, fine adjustment of curvature cannot be performed for the part where a rounded deviation of frequency is required. Hence, an ideal compensation control cannot be performed, or in other words, an ideal compensation curve cannot be obtained, in the compensation of a temperature characteristic 62 of the crystal resonator X, and the precision of the temperature characteristic after the temperature compensation is ±2 ppm.
In the technology conceivably provided above, since the non-linear capacitance deviation of the MOS varactor is similar to a curve expressed by a cubic function, the temperature compensation may be performed in a manner where the voltage applied to the MOS varactor behaves like a simple linear function. However, since the compensation curve therein is not ideal, the precision of the temperature compensation is not high. It is ideal that the MOS varactor MH in a high temperature (high-temperature compensation MOS capacitor MH), does not have frequency sensitivity in a low temperature. However, in actuality, there is a slight frequency sensitivity, and the control voltage in a high temperature (high-temperature section control voltage signal VH) has an effect in the low temperature. This involves a problem that the frequency adjustment becomes complicated, requiring, for example, to select a crystal resonator with specific frequency-temperature characteristic, in the case of products that require a high stability of frequency in a high precision.